Parallel image encoding

[fintelia]

Adds a feature flag (rayon) that enables parallel encoding of PNGs. This drastically reduces the time taken to encode a single image. With target-cpu=native, my 6 core/12 thread CPU is able to exceed 1 billion pixels/second encoding speed on the QOI benchmark suite:

                                              Ratio             Encode
---------                                    -------     --------------------
Single-threaded                              27.861%     245 mps  0.829 GiB/s
Single-threaded (target-cpu=native)          27.861%     308 mps  1.042 GiB/s

Parallel                                     27.861%     917 mps  3.099 GiB/s
Parallel (target-cpu=native)                 27.861%    1084 mps  3.664 GiB/s

Depends on image-rs/fdeflate#23.

[Shnatsel]

FWIW https://crates.io/crates/mtpng exists and can be used as inspiration/reference